Gasification refers to the conversion of a solid or liquid material, such as a carbonaceous material, into a gaseous fuel otherwise known as product gas. Gasification is of interest for many low-emission technologies in chemical and energy industries.
Gasification can be applied to a wide range of carbonaceous materials. In particular, low-rank carbonaceous fuels such as brown coal (lignite), peat, biomass and solid wastes are particularly suitable for gasification due to their high gasification reactivities. These low-rank fuels, however, are a complex mixture of organic and inorganic species and the resulting raw product gas is typically contaminated with tarry residues, fine particulates and alkali and alkaline earth metallic (AAEM) species as well as trace inorganics such as manganese, boron, copper, iron, molybdenum and zinc, and pollutant-forming species (e.g. NH3, HCN, NOx, SOx and H2S). Tarry residues in the product gas tend to condense at lower temperatures in the downstream equipment, thereby causing operational difficulties. The inorganic species may also be volatilised into the product gas and cause serious problems for the operation of downstream equipment, including corrosion/erosion of turbine/engine components used for electricity generation.
Accordingly, it is generally necessary to clean the raw product gas before it can be used, for example, as a gaseous fuel in a turbine/engine or as a feedstock for chemical synthesis. The removal of various undesirable components such as tarry materials, AAEM vapour, particulates and H2S/NH3/HCl/HCN from the gasification product gas contributes to the overall gasification process complexity and forms a significant component of the overall gasification capital and operating costs.
Scrubbing the raw product gas with a liquid, such as water or biodiesel, is a common practice to remove tarry residues and other undesirable species. However, the raw product gas must be cooled down first, causing some species to condense. It is also challenging to effectively recover heat from the raw product gas because of the unavoidable deposition of these species on the heat exchanger surface. Furthermore, the scrubbing operation merely transfers the tarry components and other undesirable species in the raw product gas into the water to create a liquid waste stream which requires expensive treatment prior to disposal. Many rural areas, where a biomass gasification system could be installed, for example, for distributed power generation, do not have a suitable source of water for the scrubbing operation. The use of other liquids such as biodiesel would be expensive, not only because of the purchasing cost but also because of the cost to transport the liquid to remote regions.
Many inorganic species in biomass are essential macro-nutrients (e.g. K, Mg and Ca) and micro-nutrients (e.g. manganese, boron, copper, iron, molybdenum and zinc) for the growth of biomass. The complex reactions involving these inorganic species during gasification may turn them into chemically very stable species such as silicates and consequently they become unusable or less suitable for the growth of new biomass even if the ash is returned back to the land. Alternatively, they may volatilise and end up as sludge (after the scrubbing operation) or reside in other forms of wastes which cannot be returned to the field. The loss of these nutrients from the land will deteriorate the long-term productivity and sustainability of agricultural land. In fact, it has been suggested that a significant fraction of biomass costs will be associated with additional costs to buy fertilisers to replenish the nutrients lost from the land due to the use of biomass as an energy source. Unfortunately, not all nutrients are replenished by these fertilisers. Therefore, there is a need to ensure that these nutrients can be retained during gasification in the forms that are accessible to the new biomass growth and returned back to the field. This is important for reducing the costs of biomass feedstock and ensuring the long-term sustainability of bioenergy as a green renewable energy source.
There is therefore a need for technological advancement.
Any references to background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the apparatus and process as disclosed herein.